The invention relates generally to a thermal transfer ink and more particularly to a thermal transfer ink and transfer medium for use with a thermal transfer printing apparatus in which selected variations in the heat generated by a thermal print head of the printing apparatus can create selected gradations in print density of printed dots of ink.
Thermal transfer ink media for use with conventional thermal transfer printers commonly include a uniform ink layer disposed on a heat resistant support layer. Conventional thermal transfer printing and thermal transfer ink media are discussed in U.S. Pat. No. 4,503,095, the contents of which are hereby incorporated herein by reference. Heat from a thermal print head corresponding to a printing signal selectively melts portions of the ink layer and transfers dots of ink from the transfer medium to a recording medium, such as paper, in a sensitive manner with high reproducibility.
Conventional thermal transfer inks typically include a binder having a melting point in the range of between about 50.degree. C. to 100.degree. C., or about the same melting point range of the ink and are compatible therewith for satisfying the performance requirements of the ink. The ink typically includes a colorant, such as a pigment or dye which is mixed with the binder such as by heat kneading and the mixture is disposed on a support by hot melt coating, solvent dispersion, solvent coating or emulsion coating.
Because conventional inks typically melt in the range of 50.degree. C. to 100.degree. C., conventional thermal transfer inks and thermal transfer ink media have inadequate resistance to prolonged exposure to high temperature and high humidity. This can lead to undesirable softening of the ink layer and to the undesirable blocking phenomenon and other undesirable staining.
When conventional multi-color ink transfer media are prepared, such as three color ink media that include yellow, magenta and cyan color bands for printing full color images by successive thermal transfer, the print density gradation of the transferred ink is inadequate. As a result, too much or too little ink is transferred and there is no adequate method for controlling color gradations and densities. This leads to unclear images and an inadequate range of colors. Thus, these conventional thermal transfer inks and thermal transfer ink media are not fully satisfactory.
Accordingly, it is desirable to provide a thermal transfer ink and ink media which avoid the shortcomings of the prior art and can provide clear uniform characters, broad color reproducing range when printing color images by the successive multi-color method and images with superior color gradations while resisting the deleterious effects of exposure to high heat and high humidity.